1. Field of the Invention
The present invention relates to a radiation pulse energy control system, a lithographic apparatus and a method for manufacturing a device.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate or part of a substrate. A lithographic apparatus can be used, for example, in the manufacture of flat panel displays, integrated circuits (ICs) and other devices involving fine structures. In a conventional apparatus, a patterning device, which can be referred to as a mask or a reticle, can be used to generate a circuit pattern corresponding to an individual layer of a flat panel display (or other device). This pattern can be transferred onto all or part of the substrate (e.g., a glass plate), by imaging onto a layer of radiation-sensitive material (e.g., resist) provided on the substrate.
Instead of a circuit pattern, the patterning device can be used to generate other patterns, for example a color filter pattern or a matrix of dots. Instead of a mask, the patterning device can be a patterning array that comprises an array of individually controllable elements. The pattern can be changed more quickly and for less cost in such a system compared to a mask-based system.
A flat panel display substrate is typically rectangular in shape. Lithographic apparatus designed to expose a substrate of this type can provide an exposure region that covers a full width of the rectangular substrate, or covers a portion of the width (for example half of the width). The substrate can be scanned underneath the exposure region, while the mask or reticle is synchronously scanned through a beam. In this way, the pattern is transferred to the substrate. If the exposure region covers the full width of the substrate then exposure can be completed with a single scan. If the exposure region covers, for example, half of the width of the substrate, then the substrate can be moved transversely after the first scan, and a further scan is typically performed to expose the remainder of the substrate.
Radiation sources typically used with a lithographic apparatus include pulsed laser sources. Typically, for a mask-based lithographic apparatus, excimer lasers are used and several tens of laser pulses are used to expose each pattern on a part of a substrate. A problem with excimer lasers is that there is a random variation of pulse energy of plus or minus about 10% of the intended energy for each pulse. However, by using a fast control algorithm and the fact that the exposure dose on the substrate is typically made up from about 40 to 60 pulses, the variation of the energy dose received at the substrate is typically of the order of plus or minus about 0.1% or below.
In a maskless apparatus, because the size of the image projected onto the substrates at any one instant is relatively small, and in order to provide an adequate throughput of substrate through the lithographic apparatus, the pattern set by the patterning device can be imaged onto the substrate using a single pulse of the radiation system. However, for a single pulse, as discussed above, the energy variation can be plus or minus about 10%. Such a variation in the energy of the pulse results in an unacceptably high variation in the line width produced on the substrate. Even if two or three pulses of radiation are used to expose each image, sufficient dose energy control (which can be required to be plus or minus about 0.5% or better) is not attainable.
To date, excimer lasers with improved pulse energy stability have not been produced. It has therefore previously been proposed to trim the energy in each pulse of radiation by a variable amount, such that the resulting pulses of radiation have less variation. In order to achieve this, it has been proposed to use a “fast” detector to detect the energy in an input pulse of radiation, an optical delay line, and a “fast” optical shutter to trim a portion of the energy from the pulse. For a viable system, it has been proposed that both the “fast” detector and the optical shutter have nano-second response times. For the optical shutter, it has been proposed to use a Pockels cell that uses an electro-optic material. However, a suitable electro-optic material is difficult to identify. In particular, the material must have high radiation transmission (in order to maximize the radiation dose available for imaging and in order to avoid heating of the Pockels cell), a long reliable lifetime, and a high switching speed such that the optical shutter can trim a portion of the pulse of radiation. As an additional requirement, all of these capabilities must be provided for use with radiation at a wavelength that is suitable for the lithographic process, for example about 193 nm.
Therefore, what is needed is a system and method for providing pulses of radiation suitable for use in lithography with improved radiation pulse energy consistency.